Two or more species having a close ecological relationship evolve together such that one species adapt to the changes of the other, thereby affecting each other’s evolution.
For instance, at microscopic level, the genes of interdependent species are found to evolve together. At macroscopic level, the traits of interdependent species co-vary over time. For example is the coevolution of insects and flowers they pollinate or the coevolution between predators and their prey.
Word origin: co-, together + evolution.